1. Field of the Invention
The present invention relates to an image coding apparatus for coding a color image signal used in a color facsimile device or the like, and also to an image decoding apparatus for decoding a coded image signal.
2. Description of the Prior Art
For instance, in a color facsimile system, red, green and blue color image signals are produced by reading a color original by utilizing a color image scanner, and then these color image signals are transmitted through a communication line, or channel such as a telephone line. Generally speaking, when these color image signals are transmitted, signal coding operation is carried out so as to reduce the amount of transmission data. Similarly, this signal coding operation is performed when an image signal is stored into a signal storage apparatus. In particular, since the amount of color image data is greater than that of monochrome image data, an effective image coding method is necessarily required.
Then, various sorts of image coding methods have been proposed. For example, as the color still picture coding method, in the JPEG (Joint Photographic Expert Group) jointed ISO with CCITT, there is a color still picture coding system, taking account of the international standardization. This coding system is described in the lecture on PCSJ image coding system held in 1990, "Trend on International Standardization for Color Still Picture" by Ohmachi, pages 43 to 48. In this coding system, the input image is processed by the discrete cosine transform (DCT). The transformed input image is further quantized and then entropy-coded to output compressed data. The compressed data is entropy-decoded. The entropy-decoded data is further inverse-quantized and then inverse-discrete cosine transformed, so that a decoded image is outputted.
In FIG. 9, there is shown an arrangement of an image coding apparatus in this image coding method with employment of a color component such a density color difference system and a luminance color difference system.
In the arrangement shown in FIG. 9, reference numeral 81 shows an input image signal, reference numeral 82 denotes a color component converter for converting a color component of the input image signal 81, and reference numerals 831, 832, 833 represent color components converted by the color component converter 82. Furthermore, reference numeral 84 indicates a segmenting (blocking) circuit for dividing the respective color components 831, 832, 833 into blocks made of m.times.n pixels, reference numeral 85 shows a coding device (encoder) for coding the images segmented by the segmenting circuit 84 with respect to each color component, and reference numeral 86 denotes either an image storage apparatus, or a transmission path, reference numeral 87 indicates a decoder for decoding the segmented images with respect to the respective color image components, and also reference numeral 88 shows a decoded image signal.
A description will now be made of an operation of the image coding apparatus. The input image signal 81 is input into the color component converter 82 to be converted into a color component of either the density color difference system, or the luminance color difference system. For example, as schematically illustrated in FIG. 5, when the input image signal 81, which has been input with the RGB color difference system, is converted by the color component converter 82 into the Lab color difference system corresponding to the density color difference signal system, the color component converter 82 executes a process as shown in the following formula: ##EQU1## where a.sub.ij (i, j=0, 1, 2) is a conversion coefficient, R, G, B are color components of the RGB color difference system, and L, a, b are color components of the Lab color difference system. As represented in FIG. 6, all of these color difference components 831, 832 and 833 are divided by the segmenting circuit 84 into blocks constructed of m.times.n pixels, and furthermore coded by the coding device 85, so that the resultant coded color segments are transferred to either the transmission path, or the image storage apparatus 86. For instance, to increase the coding efficiency in the coding device 85, the discrete cosine transformation (DCT) as defined in the following formula is performed with respect to the respective blocks subdivided by the above-described segmenting circuit 84 in case of m=n=N: ##EQU2## where C(w) (=C(u), C(v)) is defined as follows: ##EQU3##
In the above-described formula, symbol f(i,j) indicates an image signal within the blocks subdivided by the segmenting circuit 84, and F(u,v) represents a discrete cosine transform coefficient for transforming the above-explained image signal f(i,j). Assuming now that "C" is a matrix of N.times.N in which F(u,v) is a component, "A" is a coefficient matrix of N.times.N, and "X" is a matrix of N.times.N in which f(i,j) is a component, this discrete cosine transformation is equivalent to the below-mentioned formula, and a product-summation calculation must be carried out 2N.sup.3 times for each block subdivided by the segmenting circuit 84. That is to say, the calculation must be performed N.sup.3 times so as to calculate a product A.sup.t X between X and the matrix A.sup.t of N.times.N. Furthermore, since the product calculation of the matrix must be executed twice in order to calculate A.sup.t XA, a total calculation amounts to 3N.sup.3. EQU C=A.sup.t XA,
where "t" represents a transposed matrix. Also, a component "a.sub.ij " of A is shown by the below-mentioned formula: ##EQU4##
In addition, in the decoding device 87, data which has been transferred from either the transmission path, or the image storage apparatus 86 is decoded, so that the decoded image signal 88 is obtained. In this decoding device 87, the following inverse discrete cosine transformation (IDCT) process is performed: ##EQU5##
Similarly, the product summation calculation must be carried out 2N.sup.3 times with regard to the respective blocks subdivided by the segmenting circuit 84, even in performing this inverse discrete cosine transformation.
In accordance with the above-described conventional method, the discrete cosine transformation and the inverse discrete cosine transformation are required every time the color difference components are transferred, or stored. Even in case that the color difference component within the block subdivided by the segmenting circuit 84 becomes 0 (hereinafter this case will be referred to a "monochrome", whereas a not "monochromatic" case will be called as a "color"), both of the discrete cosine transformation process and the inverse discrete cosine transformation process are required which give a heavy load to the coding device and the decoding device, although these processes are unnecessary when all or part of the color difference component is monochrome. To execute the above-described inverse discrete cosine transformation, at least one discrete cosine transformation coefficient must be transferred, or stored. Accordingly, it is difficult to increase the coding efficiency rather than 1/N.sup.2 if no consideration is made of the entropy coding operation, resulting in a drawback.